The nef gene product of the human immunodeficiency virus type 1 (HIV-1), p27, has been implicated in the establishment of latent infection through down-regulation of virus replication. We have found that expression of p27 from HIV-lSF2 in lymphoid cells suppresses the replication of some HIV-1 and HIV-2 isolates, but not the growth of fast replicating and cytopathic viral strains. This differential susceptibility of HIV strains to nef suppression defines another heterogeneous property of HIV, and has implications for viral pathogenesis in the host. This proposal is aimed at further examining the biologic and molecular features of nef with the objective of elucidating the mechanism(s) by which nef can affect viral replication. The mechanism(s) underlying non-responsiveness of some viral strains to the negative effect of nef will also be investigated. We will use stable lymphoid cell lines that express HIV-lSF2 nef and viral isolates that show differential susceptibility to nef gene suppression in order to examine whether there are any functional differences in p27 (Specific Aim #1) or in the viral sequences responsive to p27 (Specific Aim #2). The functional domains of p27 will be examined (Specific Aim #3), with particular regard to the association of its GTP-binding and phosphorylation properties with its HIV inhibitory effects. Furthermore, we will study the interaction of nef with positive viral regulatory elements (tat, rev) in determining the outcome of an HIV infection (Specific Aim #4). Finally, the possibility that p27 effects are mediated through cellular proteins (Specific Aim #5), or interfere with the transduction of T-cell activation signals (Specific Aim #6) will be investigated. These studies are undertaken in hopes that elucidating the function of nef could lead to new approaches of anti-HIV therapy.